Conventionally, public wireless communication systems such as LTE (Long Term Evolution) can provide a variety of services to users through packet access. In such public wireless communication systems, the required information rate, delay, and others vary among services. The public wireless communication systems therefore prepare a plurality of classes depending on QoS (Quality of Service) and set a proper bearer for each service. FIG. 21 is a diagram illustrating classification in LTE. Referring to FIG. 21, nine classes are prepared in LTE.
The field of MTC (Machine Type Communication) has recently attracted attention, in which machines perform communication (machine communication) with each other without involving user's operation. MTC finds a wide variety of applications including security, medical care, agriculture, factory automation, and life line control. Among the applications of MTC, in particular, smart grids have attracted attention, which allow efficient transmission and distribution of energy by integrating, for example, information of electric power measured by a measurer called a smart meter, as illustrated in NPD 1 below.
Communications between MTC devices and between an MTC server managing MTC devices and an MTC device are expected to increasingly grow in the future. At present, as described in NPD 2, studies have been carried out to apply a system using a 3GPP (Third Generation Partnership Project) network such as LTE or a system using a short-range communication system in accordance with the IEEE 802.15 standard, to such communications.
MTC involves an extremely large number of devices and thus may require an enormous amount of control signals. In this respect, NPD 2 below proposes a grouping-based MTC management method. In this MTC management method, MTC devices that require various QoS are grouped according to permissible values of QoS, and AGTI (Access Grant Time Interval) corresponding to each group is allocated to each MTC device.
As a communication system for MTC devices, for example, the IDMA (Interleave Division Multiple Access) system is drawing attention, as described in NPD 3. According to NPD 3, the advantages of using the IDMA system in MTC communications include eliminating the need for scheduling and effectively applying a multi user interference canceller.
The signal receiving and demodulating processing in the IDMA system will be described below. For a channel in mobile communication, it is particularly effective to use a system called OFDM-IDMA, which uses IDMA and OFDM (Orthogonal Frequency Division Multiplexing) in combination. NPD 4 below explains the principle of the OFDM-IDMA. FIG. 22 is a diagram illustrating the principle of the OFDM-IDMA.
Referring to FIG. 22, each MTC device of each user encodes data to be transmitted with an encoder. Each MTC device then interleaves the encoded data with an interleaver. Each MTC device then modulates the interleaved signal. Each MTC device then performs inverse discrete Fourier transform of the modulated signal. A transmission signal is thus generated in each MTC device. An encoder common to the MTC devices is used. An interleaver different among devices is used.
The signal input to the antenna of a base station device is a mixture of signals from a plurality of MTC devices. The signal input to the antenna of the base station device additionally includes noise and interference. The base station device performs discrete Fourier transform of the signal. The base station device then performs MUD (Multi User Detection) on the signal obtained by discrete Fourier transform. The base station device thus separates the received signal into signals of individual users. MUD extracts a signal component of each user from the signal including a mixture of signals from a plurality of users. MUD adopts a method of gradually reducing interference components through iterative processing for the IDMA signal.
FIG. 23 is a diagram illustrating the operation of MUD. Referring to FIG. 23, the signal DFT-processed in the base station device is sent to an ESE (Elementary Signal Estimator). The ESE obtains the mean and variance for each bit, using Gaussian approximation. The ESE sends the means and variance to a deinterleaver corresponding to the interleaver of each user. The deinterleaver sends the deinterleaved signal (output) to an APP (A Posteriori Probability) decoder. The APP decoder performs decoding of a received sequence of log-likelihoods of channel bits, outputs the decoding result as a decoded signal for each user, and encodes it again for output to the interleaver with improved accuracy of the log-likelihood information. The ESE re-calculates the mean and variance based on the likelihood information of the transmission signal of each user that is sent from each APP decoder. MUD iteratively performs the processing above to increase the accuracy of signal estimation.
Japanese Patent Laying-Open No. 2007-60212 (PTD 1) discloses a configuration using a relay (relay device, repeater) that relays transmission data in uplink communication between a base station device and a portable terminal device.
NPD 5 below describes global standardization trends of cellular technology applied to machine communication.